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To mirror a pose, i usually set scale to -100% and xrot to 180 but that’s not working properly with the python either (the normals get flipped). Maybe it’s the way the python is set up, i don’t know (still a newbie too).
I did find a quick fix with Hexagon, Daz’s modelling software. If you don’t have it already, you can get it for free here : Hexagon 2.5

Here’s how i did it : select the python you want to mirror and send it to Hexagon (in Daz Studio menu : File / Send to Hexagon…)
This will open Hexagon with the python loaded. Select the pyhton, now in the properties panel set the value for Size X negative (the red box) and click Validate. Now send it back to Daz Studio (in Hexagon menu : File / Send to Daz Studio…)
Switch back to Daz Studio, it will ask for an “Action for python”. In the dropdown list for Action, choose ‘Create Prop’ and click ‘Accept’.

You now have a mirrored python in Daz. Make sure your python is set up properly before you do this, the mirrored copy can not be posed and does not have morphs anymore. You can still apply materials.

Hope this helps, i know it’s not a solution to the problem, just a workaround, but it really is just a few clicks.
Maybe someone else will pop in with a solution.